Aluminum hydroxide, also known as alumina trihydrate (ATH) or hydrated alumina, is a non-halogen flame retardant based on the chemical formula Al(OH).sub.3. It has been used in a variety of polymers, including unsaturated polyesters, elastomers, and some thermoplastics to reduce their combustibility. Aluminum hydroxide provides a flame retardant effect through its endothermic water release under heating and burning conditions. The use of aluminum hydroxide as a flame retardant has been disclosed in U.S. Pat. Nos. 4,105,465 and 4,525,494. A drawback associated with aluminum hydroxide is that high loading is often required to attain effective flame retardancy. At such high loading, the physical properties of aluminum hydroxide filled polymer compositions deteriorates substantially. Processing difficulties in manufacturing such highly loaded polymer compositions are also experienced, even in the presence of coupling agents such as organosilanes or other surface treatment agents such as fatty acids.
Melamine is also a flame retardant, mainly used in intumescent paints and polyurethane foams. Its mechanism of flame retardancy appears to involve endothermic sublimation and release of less combustible gases upon heating and burning. Due to the coarse nature of melamine particles, the melamine filled polymer compositions generally exhibit poor mechanical properties. Furthermore, the conventional dry grinding of melamine powder with ceramic media to reduce the particle size of melamine is not effective. This is due in part to the melamine powder becoming soft as mechanical energy is applied to it, thereby causing the melamine to smear onto the grinding media and, consequently, the grinding process is substantially impeded.
Thermoplastic polyolefins are widely used in consumer products, electric and electronic appliances, automobile parts, and other industrial applications. These polymers are normally flammable and can be ignited easily by flame or by overheating. The flammability of these thermoplastic polyolefins can be substantially reduced by the incorporation of flame retardants. One common approach is to use halogenated (brominated or chlorinated) additives along with antimony oxide or other metal oxides. Such systems generate hydrogen bromide (HBr) or hydrogen chloride (HCl) during the combustion of polymers to trap free radicals which are found in the flame zone. The inherent shortcoming of using halogenated additives is the evolution of dense smoke, and corrosive and toxic gases such as HBr and HCl.
The second approach to flame retarding thermoplastics is to use a hydrated mineral or surface treated hydrated mineral. A common mineral utilized is aluminum hydroxide. As stated above, a drawback of using hydrated minerals such as aluminum hydroxide is the high loadings needed to achieve effective flame retardancy which have an adverse impact on the mechanical properties of polyolefin compositions.
Surface treated aluminum hydroxide and melamine blends have been developed which require less loading to attain effective flame retardancy and thereby exhibit acceptable mechanical properties when added to polymeric materials. Further, the blends do not emit significant amounts of corrosive gases and smoke when the materials to which they are added are exposed to fire. However, since the methods for making these blends are inefficient, they have not heretofore been effectively introduced into thermoplastic polyolefin materials such as polypropylenes and polyethylenes.